Render Image without background
After some trial and error in the Compositing tab I figured out thatdisconnecting the movie clip from the render layer removed that clip, obviously, but the background then turned white, after which I connected both the Composite and Viewer Alpha channels to the Render Layer, after which the background dissapeared, just like I wanted.

So after rendering with the above node setup my result was just a white mesh without textures or any surface an a transparent background because the render window was somehow overlaying the fore and background. I then tried some other Node conection variations, until I ended up with this one:

This one got me the result I wanted, the mesh, alone and with a transparent background. I don't know what exactly happened, but this seems to do the job for me.